EP0548842A1 - Device for determining of moisture content of gases - Google Patents

Device for determining of moisture content of gases Download PDF

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Publication number
EP0548842A1
EP0548842A1 EP92121594A EP92121594A EP0548842A1 EP 0548842 A1 EP0548842 A1 EP 0548842A1 EP 92121594 A EP92121594 A EP 92121594A EP 92121594 A EP92121594 A EP 92121594A EP 0548842 A1 EP0548842 A1 EP 0548842A1
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gas
measuring elements
electrolyte measuring
gases
solid electrolyte
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EP0548842B1 (en
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Hans-Josef Speicher
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Saarbergwerke AG
Yokogawa Deutschland GmbH
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Yokogawa Deutschland GmbH
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
    • G01N27/416—Systems
    • G01N27/417—Systems using cells, i.e. more than one cell and probes with solid electrolytes
    • G01N27/4175—Calibrating or checking the analyser

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  • the invention relates to a method for continuously detecting the moisture content of gases, in particular flue gases, with a changing composition using a solid electrolyte measuring element.
  • Such a gas with a constantly changing composition or oxygen concentration for.
  • the concentration of pollutants contained in flue gas requires knowledge of the moisture content to determine the standard volume. So far, the moisture content due to the combustion stoichiometry has been taken into account when determining the standard volume of the flue gas. Errors of up to 10% were accepted.
  • this object is achieved according to the invention in that the sample gas stream is divided and a first partial stream is fed directly as a sample gas to a solid electrolyte measuring element, the second partial stream is dried to a defined residual moisture and this dry sample gas is passed to a second solid - Electrolyte measuring element is supplied, reference gases of constant composition being used in each case in the solid electrolyte measuring elements, and in that the measured values of the two solid electrolyte measuring elements are applied to a computing circuit.
  • Zircon measuring elements are advantageously used as solid electrolyte measuring elements.
  • the asymmetry potentials of the zircon measuring elements can again be viewed as constants and thus eliminated by a measuring or arithmetic circuit.
  • the moisture content of the gas to be examined can then be determined from the comparison of the measured values for moist and dry sample gas.
  • the difference in the output voltages of the two zircon measuring elements can be applied directly to the arithmetic circuit, since then the asymmetry potentials C of both elements are the same and are an additive element of the Nernst Eliminate equations when forming differences.
  • the oxygen proportions of the reference gases are known, it is advisable, however, to first determine the oxygen partial pressures of the moist and dry sample gas partial stream in the usual way for analyzers assigned to gases with a constant composition and then to determine the moisture content of the in the arithmetic circuit investigating gas from the simpler relationship to determine. It is not necessary to use the same reference gas in both zircon measuring elements. In this case, it is sufficient that the oxygen partial pressures of the reference gases are known. Basically, however, it makes sense to use ambient air as the reference gas.
  • the drying of the second sample gas partial stream is advantageously carried out by cooling to a predetermined dew point temperature at which the saturation of the sample gas partial stream no longer depends on the gas composition, and the dried sample gas partial stream thus has a constant, known residual moisture which can be compensated for by measurement technology.
  • a predetermined dew point temperature at which the saturation of the sample gas partial stream no longer depends on the gas composition
  • the dried sample gas partial stream thus has a constant, known residual moisture which can be compensated for by measurement technology.
  • cooling to 2 ° C is advisable.
  • a constant pressure level should be maintained in the sample gas stream diverted from the gas to be examined. This can be achieved in a simple manner by means of a so-called water reservoir, into which a branch line leading from the sample gas line, before the sample gas flow is divided, is guided.
  • the water reserve can are fed by condensate produced during the drying of the second sample gas partial flow.
  • the method according to the invention is further explained on the basis of the measuring circuit shown as an example in the figure.
  • a flue gas is branched from the flue gas stream 1 via a blower 2, a measuring gas stream 3, the measuring gas stream being kept constant and limited by a flow monitor 6, e.g. B. to 90 l / h.
  • This sample gas stream is divided into two partial streams, the first sample gas partial stream 4 being passed directly past the measuring electrode of a zircon measuring element 7.
  • the second sample gas partial stream 5 is extracted in a sample gas cooler 9 by cooling, for example to 2 ° C., down to the known residual moisture corresponding to the temperature of 2 ° C.
  • the second sample gas partial stream 5 thus dried is now guided past the measuring electrode of a second zircon measuring element 8.
  • the reference electrodes are provided with reference gases 10 and 11, each with a constant composition, i. H. constant oxygen partial pressures.
  • a heat exchanger 12 connected in the hot, moist first measuring gas partial stream 4 ensures that flue gases, for. B. the hot sample gas stream 4 does not fall below the specific acid dew point for fossil fuels at any point in the electrode. A drop below the acid dew point would, for. B. lead to sulfur deposits.
  • the heat exchanger 12 serves to compensate for the so-called dead volume of the sample gas cooler 9. This ensures that fluctuations in the composition of the flue gas and thus the sample gas affect both zircon measuring elements 7, 8 simultaneously and the measuring electrodes of the zircon measuring elements 7, 8 are always acted upon by measuring gas with the same composition - except for the humidity.
  • the measured values 14, 15 of the zircon measuring elements 7, 8 are applied to a computing circuit 13 and the moisture content of the gas to be examined is determined.
  • the difference in the output voltages of the two zircon measuring elements 7, 8 can be applied directly to the arithmetic circuit. Then, however, the same reference gas must be used in both zircon measuring elements 7, 8, so that the asymmetry potentials cancel out when the voltage difference is formed.
  • the oxygen partial pressure of the reference gas will be known. Then, as is known in the case of gases of constant composition, the oxygen partial pressures of the moist and the dry measuring gas can be directly detected in the analyzers 16, 17 assigned to the zircon measuring elements 7, 8 and these can be applied to the arithmetic circuit 13. Because of the simpler determination equation for the moisture content when the oxygen partial pressures are known, the arithmetic circuit 13 itself can also be made simpler. With this procedure, different reference gases 10 and 11, each with a known oxygen content, could be used in the zircon measuring elements 7, 8, but generally the same reference gas, e.g. B. Use ambient air.
  • a correction of the residual moisture of the dried measuring gas partial stream 5 is also integrated in accordance with the given dew point temperature.
  • the dew point temperature can be calculated from the volume fractions of water vapor in the flue gas. Including the flue gas temperature detected via sensor 18, a continuous determination of the relative humidity is also possible. With knowledge of the moisture in the flue gas, including a continuous SO3 measurement, there is the computational possibility of continuously determining the acid dew point.
  • a steam generator can also trigger a warning signal that indicates the risk of a so-called.
  • the pressure level in the sample gas stream 3 and the sample gas partial streams 4, 5 is kept constant in the example shown via a water reservoir 19, into which a branch line 20 branching off from the sample gas line is guided. If necessary, too much sample gas drawn in by the blower 2 can escape via this branch line 20.
  • the template 19 is fed with condensate 21 flowing out of the measuring gas cooler 9.
  • a zero point gas 22 or an ed point gas 23, each with a defined oxygen fraction are fed to the measuring electrodes.
  • the zero point gas 22 z. B. nitrogen with 1% by volume oxygen as the end point gas 23 nitrogen with an oxygen proportion corresponding to the required measuring range of z. B. 25 vol% can be used.
  • a minimum proportion of oxygen, even in the zero point gas 22, is necessary for the function of the zircon measuring elements. However, it can be compensated for in terms of circuitry.
  • the invention is not restricted to the use of zircon measuring elements or solid electrolyte measuring elements. Rather, it is applicable to other potentiometric measuring elements for determining oxygen partial pressure differences.

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Abstract

In a method for the continuous determination of the moisture content of gases, especially of flue gases, whose composition varies, which method employs a solid-electrode measuring element, one of whose electrodes is subjected to test gas shunted off from the gas stream to be measured and whose second electrode is subjected to a reference gas, the test gas stream is divided and a first part stream is fed directly as a test gas to a solid-electrolyte measuring element, while the second part stream is dried to a defined residual humidity and said dry test gas is fed to a second solid-electrolyte measuring element. In each of the solid-electrolyte measuring elements, reference gases having a constant composition are employed. The measured values of the two solid-electrolyte measuring elements are injected into an arithmetic circuit. If the same reference gas is employed in both the solid-electrolyte measuring elements, it is possible to inject directly into the arithmetic circuit the difference of the output voltages of the solid-electrolyte measuring elements. If the oxygen proportions of the reference gases are known, it is possible first to determine, in analysers assigned to the individual solid-electrolyte measuring elements, the oxygen partial pressures of the humid and the dry test gas part stream and then to inject these into the arithmetic circuit. The solid-electrolyte measuring elements used are expediently zirconium elements.

Description

Die Erfindung betrifft ein Verfahren zum kontinuierlichen Erfassen des Feuchtegehaltes von Gasen, insbesondere von Rauchgasen, mit sich ändernder Zusammensetzung unter Verwendung eines Feststoff-Elektrolyt-Meßelementes.The invention relates to a method for continuously detecting the moisture content of gases, in particular flue gases, with a changing composition using a solid electrolyte measuring element.

Es ist bekannt, z.B. Zirkon-Meßelemente zum Messen des Feuchtegehaltes eines Gases mit, bis auf den Feuchtegehalt selbst, gleichbleibender Zusammensetzung,
d. h. gleichbleibender Sauerstoffkonzentration einzusetzen. Zirkon-Meßelemente bestehen im wesentlichen aus von zwei beheizten Platinelektroden, der Referenzelektrode und der Meßelektrode, eingefaßter Zirkonkeramik. Bei der Verwendung als Feuchte-Messer wird die Eigenschaft von Zirkonkeramik genutzt, sich bei hoher Temperatur gegenüber Sauerstoffionen wie ein fester Elektrolyt zu verhalten. Weisen die jeweils an der Meß- bzw. Referenzelektrode vorbeigeführten Gase unterschiedliche Sauerstoffpartialdrücke auf, so baut sich zwischen beiden Elektroden eine entsprechende elektrische Spannung auf, für die gemäß der Nernst'schen Gleichung die Beziehung gilt

Figure imgb0001

Dabei bedeuten:

E:
die elektrische Spannung
y:
die Sauerstoffkonzentration (Vol-%-O₂) an der Meßelektrode
d:
die Sauerstoffkonzentration (Vol-%-O₂) an der Referenzelektrode
R:
Gaskonstante
T:
absolute Temperatur
n:
Wertigkeit des Sauerstoffmoleküls (= 4)
F:
Faraday'sche Konstante
C:
Asymmetrie-Konstante (fertigungsbedingt)
Wird eine Elektrode mit einem Referenzgas mit definiertem O₂-Gehalt, z. B. Luft mit 21 Vol-%-O₂, und die zweite Elektrode mit einem Meßgas beaufschlagt, stellt die zwischen den Elektroden gemessene Spannung unmittelbar ein Maß für die Sauerstoffkonzentration des Meßgases dar. Ändert sich bei sonst gleichbleibender Zusammensetzung der Feuchtegehalt des Meßgases, so ändert sich aufgrund der geänderten volumetrischen Zusammensetzung auch die Sauerstoffkonzentration des Meßgases und damit die zwischen den Elektroden gemessene Spannung. Die Spannungsänderung ist demnach ein Maß für die Änderung des Feuchtegehaltes im Meßgas bzw. bei entsprechendem Abgleich der Meßschaltung für dessen Feuchtegehalt selbst.It is known, for example, zircon measuring elements for measuring the moisture content of a gas with, apart from the moisture content itself, a constant composition,
ie use constant oxygen concentration. Zircon measuring elements essentially consist of two heated platinum electrodes, the reference electrode and the measuring electrode, surrounded by zircon ceramic. When used as a moisture meter, the property of zirconium ceramic is used to behave like a solid electrolyte at high temperatures towards oxygen ions. If the gases that are led past the measuring or reference electrode each have different oxygen partial pressures, a corresponding electrical voltage builds up between the two electrodes, for which the relationship applies according to Nernst's equation
Figure imgb0001
Mean:
E:
the electrical voltage
y:
the oxygen concentration (vol% O₂) at the measuring electrode
d:
the oxygen concentration (vol% O₂) at the reference electrode
R:
Gas constant
T:
absolute temperature
n:
Valence of the oxygen molecule (= 4)
F:
Faraday's constant
C:
Asymmetry constant (due to production)
If an electrode with a reference gas with a defined O₂ content, for. B. air with 21 vol.% - O₂, and the second electrode is charged with a measuring gas, the voltage measured between the electrodes directly represents a measure of the oxygen concentration of the measuring gas. If the composition of the measuring gas changes otherwise, the moisture content of the measuring gas changes, so changes due to the changed volumetric composition, the oxygen concentration of the measuring gas and thus the voltage measured between the electrodes. The change in voltage is therefore a measure of the change in the moisture content in the sample gas or, if the measuring circuit is appropriately adjusted, for its moisture content itself.

Zur Bestimmung des Feuchtegehaltes von Gasen mit sich ändernder Zusammensetzung, deren Sauerstoffkonzentration also nicht ausschließlich durch den Feuchtegehalt, sondern auch durch Änderung der Volumenanteile weiterer Gaskomponenten beeinflußt wird, können Zirkon-Meßelemente jedoch nicht ohne weiteres eingesetzt werden.To determine the moisture content of gases with a changing composition, the oxygen concentration of which is therefore influenced not only by the moisture content but also by changing the volume fractions of other gas components, zircon measuring elements cannot be used without further ado.

Ein solches Gas mit sich ständig ändernder Zusammensetzung bzw. Sauerstoffkonzentration stellt z. B. Rauchgas aus Feuerungsanlagen dar. Für die Ermittlung und Überwachung der im Rauchgas enthaltenen Schadstoffkonzentration ist jedoch die Kenntnis des Feuchtegehaltes zur Bestimmung des Normvolumens notwendig. Bisher wurde bei der Bestimmung des Normvolumens des Rauchgases der Feuchtegehalt aufgrund der Verbrennungsstöchiometrie als konstant eingerechnet. Dabei wurden Fehler von bis zu 10 % in Kauf genommen.Such a gas with a constantly changing composition or oxygen concentration, for. B. flue gas from combustion plants. For the determination and monitoring of im The concentration of pollutants contained in flue gas requires knowledge of the moisture content to determine the standard volume. So far, the moisture content due to the combustion stoichiometry has been taken into account when determining the standard volume of the flue gas. Errors of up to 10% were accepted.

Zur Einhaltung der neuen, durch das Immissionsschutzgesetz vorgeschriebenen Emissionsgrenzwerte ist es jedoch notwendig geworden, die Rauchgasfeuchte kontinuierlich schnell und exakt zu erfassen. Auch zur exakten Einstellung der Luftzahl λ bei Primärentstickung durch Rauchgasrückführung zur Verbrennungsluft ist die genaue Kenntnis des Feuchtegehaltes erforderlich.However, in order to comply with the new emission limit values prescribed by the Immission Control Act, it has become necessary to continuously and quickly record the flue gas moisture. Precise knowledge of the moisture content is also required for the exact setting of the air ratio λ in the case of primary denitrification by returning flue gas to the combustion air.

Zur Bestimmung des Feuchtegehaltes von Gasen mit sich ändernder Zusammensetzung, wie eben Rauchgase mit sich ändernden CO- und CO₂-Anteilen, wurde daher auch schon vorgeschlagen (DE-OS 36 37 218), aus dem Meßgasstrom kontinuierlich Meßgas abzuzweigen, auf eine definierte Restfeuchte zu trocknen und das nunmehr trockene Meßgas als Vergleichsgas einzusetzen, so daß die Zusammensetzung des Vergleichsgases, bis eben auf den Feuchtegehalt, zu jedem Zeitpunkt der Zusammensetzung des Meßgases entspricht. Dabei wurde von der Annahme ausgegangen, daß nunmehr eine Änderung der Volumenanteile der übrigen Gaskomponenten keinen Einfluß mehr auf das Meßergebnis hat und somit durch das Verhältnis der Sauerstoffpartialdrücke bzw. durch die Elektrodenspannung unmittelbar der Feuchtegehalt des unbehandelten Meßgases unter Berücksichtigung der Beziehung

Figure imgb0002

bestimmt werden kann nach
Figure imgb0003
Darin bedeuten

PO2tr:
Sauerstoffpartialdruck des trockenen Meßgases
PO2ft:
Sauerstoffpartialdruck des feuchten Meßgases
MBH2O:
Feuchtegehalt des Meßgases in Vol-%
Es hat sich jedoch gezeigt, daß auch dieses Verfahren noch zu ungenaue und unbefriedigende Meßergebnisse liefert. Es wurde gefunden, daß die diesem Verfahren zugrundeliegende Annahme, wonach das Asymmetriepotential C, das heißt die unter der Bedingung Referenzgas = Meßgas bzw. gleicher Sauerstoffpartialdrücke an Referenz- und Meßelektrode von einem Zirkon-Meßelement abgegebene Spannung, eine Konstante ist, erst für Sauerstoffkonzentrationen ab bzw. oberhalb etwa 18 bis 20 Vol-% gilt. Sinkt die Sauerstoffkonzentration jedoch unter diesen Wert, so steigt das Asymmetriepotential mit sinkender Sauerstoffkonzentration immer steiler an, wobei die Abhängigkeitskurve einen in etwa hyperbolischen Verlauf zeigt.For the determination of the moisture content of gases with a changing composition, such as flue gases with changing CO and CO₂ proportions, it has therefore already been proposed (DE-OS 36 37 218) to branch off measuring gas continuously from the sample gas stream to a defined residual moisture dry and use the now dry sample gas as reference gas so that the composition of the reference gas corresponds to the composition of the sample gas at all times except for the moisture content. The assumption was made that a change in the volume fractions of the other gas components no longer has any influence on the measurement result, and thus the moisture content of the untreated sample gas, taking into account the relationship, due to the ratio of the oxygen partial pressures or the electrode voltage
Figure imgb0002
can be determined according to
Figure imgb0003
Mean in it
P O2tr :
Partial oxygen pressure of the dry sample gas
P O2ft :
Partial oxygen pressure of the moist sample gas
MB H2O :
Moisture content of the sample gas in vol%
However, it has been shown that this method too still gives measurement results which are still too imprecise and unsatisfactory. It was found that the assumption on which this method is based, according to which the asymmetry potential C, that is to say the voltage emitted by a zirconium measuring element under the condition of reference gas = measuring gas or the same oxygen partial pressures at the reference and measuring electrodes, is only a constant for oxygen concentrations or above about 18 to 20% by volume. However, if the oxygen concentration falls below this value, the asymmetry potential increases with increasing oxygen concentration, the dependency curve showing an approximately hyperbolic course.

Aufgabe der vorliegenden Erfindung ist es daher, ein verbessertes Verfahren zum Erfassen des Feuchtegehaltes von Gasen anzugeben, das in einfacher Weise ein genaues kontinuierliches Bestimmen des Feuchtegehaltes von Gasen mit sich ändernder Zusammensetzung ermöglicht.It is therefore an object of the present invention to provide an improved method for detecting the moisture content of gases, which enables simple, precise, continuous determination of the moisture content of gases with a changing composition.

Bei einem Verfahren der eingangs genannten Art wird diese Aufgabe erfindungsgemäß dadurch gelöst, daß der Meßgasstrom geteilt und ein erster Teilstrom unmittelbar als Meßgas einem Feststoff-Elektrolyt-Meßelement zugeführt wird, daß der zweite Teilstrom auf eine definierte Restfeuchte getrocknet und dieses trockene Meßgas einem zweiten Feststoff-Elektrolyt-Meßelement zugeführt wird, wobei in den Feststoff-Elektrolyt-Meßelementen jeweils Referenzgase konstanter Zusammensetzung verwendet werden, und daß die Meßwerte der beiden Feststoff-Elektrolyt-Meßelemente einer Rechenschaltung aufgeschaltet werden.In a method of the type mentioned, this object is achieved according to the invention in that the sample gas stream is divided and a first partial stream is fed directly as a sample gas to a solid electrolyte measuring element, the second partial stream is dried to a defined residual moisture and this dry sample gas is passed to a second solid - Electrolyte measuring element is supplied, reference gases of constant composition being used in each case in the solid electrolyte measuring elements, and in that the measured values of the two solid electrolyte measuring elements are applied to a computing circuit.

Als Feststoff-Elektrolyt-Meßelemente werden vorteilhafterweise Zirkon-Meßelemente eingesetzt.Zircon measuring elements are advantageously used as solid electrolyte measuring elements.

Durch die Verwendung z.B. zweier Zirkon-Meßelemente und den getrennten Vergleich von feuchtem und trockenem Meßgas gegenüber jeweils Referenzgasen mit konstantem Sauerstoffanteil können die Asymmetriepotentiale der Zirkon-Meßelemente jeweils wieder als Konstanten angesehen und somit durch eine Meß- bzw. Rechenschaltung eliminiert werden. Aus dem Vergleich der Meßwerte für feuchtes und trockenes Meßgas kann dann der Feuchtegehalt des zu untersuchenden Gases bestimmt werden.By using e.g. two zircon measuring elements and the separate comparison of moist and dry measuring gas compared to reference gases with constant oxygen content, the asymmetry potentials of the zircon measuring elements can again be viewed as constants and thus eliminated by a measuring or arithmetic circuit. The moisture content of the gas to be examined can then be determined from the comparison of the measured values for moist and dry sample gas.

Werden gemäß einem weiteren Merkmal der Erfindung in beiden Zirkon-Meßelementen das gleiche Referenzgas verwendet, so kann der Rechenschaltung unmittelbar die Differenz der Ausgangsspannungen der beiden Zirkon-Meßelemente aufgeschaltet werden, da dann die Asymmetriepotentiale C beider Elemente gleich sind und sich als additives Glied der Nernst'schen Gleichungen bei der Differenzenbildung aufheben.If, according to a further feature of the invention, the same reference gas is used in both zircon measuring elements, the difference in the output voltages of the two zircon measuring elements can be applied directly to the arithmetic circuit, since then the asymmetry potentials C of both elements are the same and are an additive element of the Nernst Eliminate equations when forming differences.

Für die Differenz der Ausgangsspannungen der Zirkon-Meßelemente für das feuchte und das trockene Meßgas ergibt sich dann die Beziehung

Figure imgb0004

und das daraus wieder mit der Beziehung
Figure imgb0005
für den Feuchtegehalt des zu untersuchenden Gases
Figure imgb0006
Da der Verlauf der Abhängigkeitskurve des Asymmetriepotentiales vom Sauerstoffpartialdruck des Referenzgases in gewissen Grenzen auch noch von der Dotation der Zirkon-Keramik abhängt, ist es zweckmäßig, jeweils Zirkon-Meßelemente derselben Fertigungsserie zu verwenden, um möglichst exakte Ergebnisse zu gewährleisten.The relationship then arises for the difference between the output voltages of the zircon measuring elements for the moist and the dry measuring gas
Figure imgb0004
and that again with the relationship
Figure imgb0005
for the moisture content of the gas to be examined
Figure imgb0006
Since the curve of the dependency curve of the asymmetry potential on the oxygen partial pressure of the reference gas also within certain limits also on the doping of the zirconium ceramic depends, it is advisable to use zircon measuring elements from the same production series in order to ensure the most exact results possible.

Sind die Sauerstoffanteile der Referenzgase bekannt, ist es jedoch zweckmäßig, zunächst die Sauerstoffpartialdrücke jeweils des feuchten und des trockenen Meßgas-Teilstromes in der bei Gasen konstanter Zusammensetzung üblichen Weise durch den Zirkon-Meßelementen zugeordnete Analysatoren zu bestimmen und danach in der Rechenschaltung den Feuchtegehalt des zu untersuchenden Gases aus der einfacheren Beziehung

Figure imgb0007

zu ermitteln. Dabei ist es nicht erforderlich, daß in beiden Zirkon-Meßelementen das gleiche Referenzgas verwendet wird. Es genügt in diesem Fall vielmehr, daß jeweils die Sauerstoffpartialdrücke der Referenzgase bekannt sind. Grundsätzlich bietet es sich jedoch an, als Referenzgas Umgebungsluft zu verwenden.If the oxygen proportions of the reference gases are known, it is advisable, however, to first determine the oxygen partial pressures of the moist and dry sample gas partial stream in the usual way for analyzers assigned to gases with a constant composition and then to determine the moisture content of the in the arithmetic circuit investigating gas from the simpler relationship
Figure imgb0007
to determine. It is not necessary to use the same reference gas in both zircon measuring elements. In this case, it is sufficient that the oxygen partial pressures of the reference gases are known. Basically, however, it makes sense to use ambient air as the reference gas.

Die Trocknung des zweiten Meßgasteilstromes erfolgt vorteilhafterweise durch Abkühlung auf eine vorgegebene Taupunkttemperatur, bei der die Sättigung des Meßgas-Teilstromes nicht mehr von der Gaszusammensetzung abhängt, der getrocknete Meßgas-Teilstrom somit eine konstante, bekannte Restfeuchte hat, die meßtechnisch kompensiert werden kann. Bei Rauchgasen beispielsweise ist eine Abkühlung auf 2 °C zweckmäßig.The drying of the second sample gas partial stream is advantageously carried out by cooling to a predetermined dew point temperature at which the saturation of the sample gas partial stream no longer depends on the gas composition, and the dried sample gas partial stream thus has a constant, known residual moisture which can be compensated for by measurement technology. For smoke gases, for example, cooling to 2 ° C is advisable.

Zur Vermeidung von Meßschwankungen sollte im vom zu untersuchenden Gas abgezweigten Meßgasstrom ein konstantes Druckniveau aufrechterhalten werden. Das kann in einfacher Weise durch eine sog. Wasservorlage erreicht werden, in die eine von der Meßgasleitung, vor Teilung des Meßgasstromes, abzweigende Stichleitung geführt ist. Die Wasservorlage kann durch bei der Trocknung des zweiten Meßgas-Teilstromes anfallendes Kondensat gespeist werden.To avoid measurement fluctuations, a constant pressure level should be maintained in the sample gas stream diverted from the gas to be examined. This can be achieved in a simple manner by means of a so-called water reservoir, into which a branch line leading from the sample gas line, before the sample gas flow is divided, is guided. The water reserve can are fed by condensate produced during the drying of the second sample gas partial flow.

Anhand der in der Figur beispielhaft dargestellten Meßschaltung wird das erfindungsgemäße Verfahren weiter erläutert.The method according to the invention is further explained on the basis of the measuring circuit shown as an example in the figure.

Zur Bestimmung des Feuchtegehaltes z. B. eines Rauchgases wird von dem Rauchgasstrom 1 über ein Gebläse 2 ein Meßgasstrom 3 abgezweigt, wobei der Meßgasstrom durch einen Strömungswächter 6 konstant gehalten und begrenzt, z. B. auf 90 l/h, wird. Dieser Meßgasstrom wird in zwei Teilströme aufgeteilt, wobei der erste Meßgas-Teilstrom 4 unmittelbar an der Meßelektrode eines Zirkon-Meßelementes 7 vorbeigeführt wird. Dem zweiten Meßgas-Teilstrom 5 wird dagegen in einem Meßgaskühler 9 durch Abkühlung beispielsweise auf 2 °C die Feuchte, bis auf die bekannte, der Temperatur von 2 °C entsprechende Restfeuchte entzogen. Der so getrocknete zweite Meßgas-Teilstrom 5 wird nun an der Meßelektrode eines zweiten Zirkon-Meßelementes 8 vorbeigeführt. Die Referenzelektroden sind mit Referenzgasen 10 bzw. 11 mit jeweils konstanter Zusammensetzung, d. h. konstanten Sauerstoffpartialdrücken, beaufschlagt.To determine the moisture content z. B. a flue gas is branched from the flue gas stream 1 via a blower 2, a measuring gas stream 3, the measuring gas stream being kept constant and limited by a flow monitor 6, e.g. B. to 90 l / h. This sample gas stream is divided into two partial streams, the first sample gas partial stream 4 being passed directly past the measuring electrode of a zircon measuring element 7. In contrast, the second sample gas partial stream 5 is extracted in a sample gas cooler 9 by cooling, for example to 2 ° C., down to the known residual moisture corresponding to the temperature of 2 ° C. The second sample gas partial stream 5 thus dried is now guided past the measuring electrode of a second zircon measuring element 8. The reference electrodes are provided with reference gases 10 and 11, each with a constant composition, i. H. constant oxygen partial pressures.

Durch einen in den heißen, feuchten ersten Meßgas-Teilstrom 4 geschalteten Wärmetauscher 12 wird einmal sichergestellt, daß bei Rauchgasen z. B. der heiße Meßgas-Teilstrom 4 an keiner Stelle der Meßkette den für fossile Brennstoffe spezifischen Säuretaupunkt unterschreitet. Eine Unterschreitung des Säuretaupunktes würde z. B. zu Schwefelablagerungen führen. Gleichzeitig dient der Wärmetauscher 12 zur Kompensation des sog. Totvolumens des Meßgaskühlers 9. Dadurch ist gewährleistet, daß sich Schwankungen in der Zusammensetzung des Rauchgases und damit des Meßgases auf beide Zirkon-Meßelemente 7, 8 gleichzeitig auswirken und die Meßelektroden der Zirkon-Meßelemente 7, 8 stets von Meßgas mit - bis eben auf die Feuchte - gleicher Zusammensetzung beaufschlagt werden.A heat exchanger 12 connected in the hot, moist first measuring gas partial stream 4 ensures that flue gases, for. B. the hot sample gas stream 4 does not fall below the specific acid dew point for fossil fuels at any point in the electrode. A drop below the acid dew point would, for. B. lead to sulfur deposits. At the same time, the heat exchanger 12 serves to compensate for the so-called dead volume of the sample gas cooler 9. This ensures that fluctuations in the composition of the flue gas and thus the sample gas affect both zircon measuring elements 7, 8 simultaneously and the measuring electrodes of the zircon measuring elements 7, 8 are always acted upon by measuring gas with the same composition - except for the humidity.

Die Meßwerte 14, 15 der Zirkon-Meßelemente 7, 8 werden einer Rechenschaltung 13 aufgeschaltet und der Feuchtegehalt des zu untersuchenden Gases ermittelt.The measured values 14, 15 of the zircon measuring elements 7, 8 are applied to a computing circuit 13 and the moisture content of the gas to be examined is determined.

Grundsätzlich kann der Rechenschaltung unmittelbar die Differenz der Ausgangsspannungen der beiden Zirkon-Meßelemente 7, 8 aufgeschaltet werden. Dann allerdings muß in beiden Zirkon-Meßelementen 7, 8 das gleiche Referenzgas verwendet werden, damit sich bei der Bildung der Spannungsdifferenz die Asymmetriepotentiale aufheben.In principle, the difference in the output voltages of the two zircon measuring elements 7, 8 can be applied directly to the arithmetic circuit. Then, however, the same reference gas must be used in both zircon measuring elements 7, 8, so that the asymmetry potentials cancel out when the voltage difference is formed.

Normalerweise wird jedoch der Sauerstoffpartialdruck des Referenzgases bekannt sein. Dann können, wie bei Gasen konstanter Zusammensetzung bekannt, in den Zirkon-Meßelementen 7,8 zugeordneten Analysatoren 16, 17 direkt die Sauerstoffpartialdrücke des feuchten und des trockenen Meßgases erfaßt und diese der Rechenschaltung 13 aufgeschaltet werden. Wegen der bei Kenntnis der Sauerstoffpartialdrücke einfacheren Bestimmungsgleichung für den Feuchtegehalt kann dann auch die Rechenschaltung 13 selbst einfacher gestaltet werden. Bei dieser Verfahrensweise könnten zwar grundsäztlich in den Zirkon-Meßelementen 7, 8 unterschiedliche Referenzgase 10 und 11 mit jeweils bekanntem Sauerstoffanteil verwendet werden, jedoch wird man in der Regel das gleiche Referenzgas, z. B. Umgebungsluft, verwenden.Usually, however, the oxygen partial pressure of the reference gas will be known. Then, as is known in the case of gases of constant composition, the oxygen partial pressures of the moist and the dry measuring gas can be directly detected in the analyzers 16, 17 assigned to the zircon measuring elements 7, 8 and these can be applied to the arithmetic circuit 13. Because of the simpler determination equation for the moisture content when the oxygen partial pressures are known, the arithmetic circuit 13 itself can also be made simpler. With this procedure, different reference gases 10 and 11, each with a known oxygen content, could be used in the zircon measuring elements 7, 8, but generally the same reference gas, e.g. B. Use ambient air.

In die Rechenschaltung 13 ist auch eine Korrektur der Restfeuchte des getrockneten Meßgas-Teilstromes 5 entsprechend der gegebenen Taupunkttemperatur integriert. Aus den Volumenanteilen des Wasserdampfes im Rauchgas kann rechnerisch die Taupunkttemperatur ermittelt werden. Unter Einbeziehung der über Fühler 18 erfaßten Rauchgastemperatur ist auch eine kontinuierliche Bestimmung der relativen Feuchte möglich.
Bei Kenntnis der Feuchte im Rauchgas besteht, unter Einbeziehung einer kontinuierlichen SO₃-Messung, rechnerisch die Möglichkeit, den Säuretaupunkt kontinuierlich zu bestimmen.In the arithmetic circuit 13, a correction of the residual moisture of the dried measuring gas partial stream 5 is also integrated in accordance with the given dew point temperature. The dew point temperature can be calculated from the volume fractions of water vapor in the flue gas. Including the flue gas temperature detected via sensor 18, a continuous determination of the relative humidity is also possible.
With knowledge of the moisture in the flue gas, including a continuous SO₃ measurement, there is the computational possibility of continuously determining the acid dew point.

Bei untypischen schnellen Änderungen bzw. bei Überschreiten eines Grenzwertes des Feuchtegehaltes des Rauchgases, z. B. eines Dampferzeugers, kann darüber hinaus ein Warnsignal ausgelöst werden, das auf die Gefahr eines sog. Rohrreißers hinweist.In the case of atypical rapid changes or when a limit value for the moisture content of the flue gas is exceeded, e.g. B. a steam generator, can also trigger a warning signal that indicates the risk of a so-called.

Das Druckniveau im Meßgasstrom 3 sowie den Meßgas-Teilströmen 4, 5 wird im gezeigten Beispiel über eine Wasservorlage 19, in die eine von der Meßgasleitung abzweigende Stichleitung 20 geführt ist, konstant gehalten. Über diese Stichleitung 20 kann ggf. auch vom Gebläse 2 zuviel angesaugtes Meßgas entweichen. Die Vorlage 19 wird mit aus dem Meßgaskühler 9 abfließendem Kondensat 21 gespeist.The pressure level in the sample gas stream 3 and the sample gas partial streams 4, 5 is kept constant in the example shown via a water reservoir 19, into which a branch line 20 branching off from the sample gas line is guided. If necessary, too much sample gas drawn in by the blower 2 can escape via this branch line 20. The template 19 is fed with condensate 21 flowing out of the measuring gas cooler 9.

Zur Kalibrierung des Meßbereiches der Zirkon-Meßelemente 7, 8 werden den Meßelektroden trockene Prüfgase, ein Nullpunktgas 22 bzw. ein Edpunktgas 23 mit jeweils definiertem Sauerstoffanteil zugeführt. Als Nullpunktgas 22 kann z. B. Stickstoff mit 1 Vol-% Sauerstoff, als Endpunktgas 23 Stickstoff mit einem dem erforderlichen Meßbereich entsprechenden Sauerstoffanteil von z. B. 25 Vol-% verwendet werden. Ein Mindest-Sauerstoffanteil auch im Nullpunktgas 22 ist für die Funktion der Zirkon-Meßelemente notwendig. Er kann jedoch schaltungstechnisch kompensiert werden.In order to calibrate the measuring range of the zircon measuring elements 7, 8, dry test gases, a zero point gas 22 or an ed point gas 23, each with a defined oxygen fraction, are fed to the measuring electrodes. As the zero point gas 22 z. B. nitrogen with 1% by volume oxygen, as the end point gas 23 nitrogen with an oxygen proportion corresponding to the required measuring range of z. B. 25 vol% can be used. A minimum proportion of oxygen, even in the zero point gas 22, is necessary for the function of the zircon measuring elements. However, it can be compensated for in terms of circuitry.

Die Erfindung ist nicht auf die Verwendung von Zirkon-Meßelementen bzw. Feststoff-Elektrolyt-Meßelemente beschränkt. Sie ist vielmehr auf andere potentiometrische Meßelemente zur Bestimmung von Sauerstoffpartialdruckdifferenzen anwendbar.The invention is not restricted to the use of zircon measuring elements or solid electrolyte measuring elements. Rather, it is applicable to other potentiometric measuring elements for determining oxygen partial pressure differences.

Claims (8)

Verfahren zum kontinuierlichen Erfassen des Feuchtegehaltes von Gasen, insbesondere von Rauchgasen, mit sich ändernder Zusammensetzung unter Verwendung eines Feststoff-Elektrolyt-Meßelementes, dessen eine Elektrode mit vom zu überwachenden Gasstrom abgezweigtem Meßgas und dessen zweite Elektrode mit einem Referenzgas beaufschlagt werden, dadurch gekennzeichnet, daß der Meßgasstrom geteilt und ein erster Teilstrom unmittelbar als Meßgas einem Feststoff-Elektrolyt-Meßelement zugeführt wird, daß der zweite Teilstrom auf eine definierte Restfeuchte getrocknet und dieses trockene Meßgas einem zweiten Feststoff-Elektrolyt-Meßelement zugeführt wird, wobei in den Feststoff-Elektrolyt-Meßelementen jeweils Referenzgase konstanter Zusammensetzung verwendet werden, und daß die Meßwerte der beiden Feststoff-Elektrolyt-Meßelemente einer Rechenschaltung aufgeschaltet werden.A method for continuously detecting the moisture content of gases, in particular flue gases, with a changing composition using a solid electrolyte measuring element, one electrode of which is supplied with measuring gas branched off from the gas stream to be monitored and the second electrode of which is supplied with a reference gas, characterized in that the sample gas stream is divided and a first partial stream is fed directly as a sample gas to a solid electrolyte measuring element, that the second partial stream is dried to a defined residual moisture and this dry measuring gas is fed to a second solid electrolyte measuring element, in the solid electrolyte measuring elements reference gases of constant composition are used in each case, and that the measured values of the two solid electrolyte measuring elements of a computing circuit are applied. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß als Feststoff-Elektrolyt-Meßelemente Zirkon-Meßelemente verwendet werden.A method according to claim 1, characterized in that zircon measuring elements are used as solid electrolyte measuring elements. Verfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß in beiden Feststoff-Elektrolyt-Meßelementen das gleiche Referenzgas verwendet und der Rechenschaltung unmittelbar die Differenz der Ausgangsspannungen der Feststoff-Elektrolyt-Meßelemente aufgeschaltet wird.Method according to Claim 1 or 2, characterized in that the same reference gas is used in both solid electrolyte measuring elements and the difference in the output voltages of the solid electrolyte measuring elements is directly applied to the computing circuit. Verfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß bei bekannten Sauerstoffanteilen der Referenzgase zunächst in den einzelnen Feststoff-Elektrolyt-Meßelementen zugeordneten Analysatoren die Sauerstoffpartialdrücke des feuchten und des trockenen Meßgas-Teilstromes bestimmt und diese der Rechenschaltung aufgeschaltet werden.Method according to Claim 1 or 2, characterized in that, in the case of known oxygen fractions of the reference gases, the oxygen partial pressures of the moist and the dry measuring gas partial flow are first determined in the analyzers assigned to the individual solid electrolyte measuring elements and these are connected to the arithmetic circuit. Verfahren nach einem der Ansprüche 1 bis 4, dadurch gekennzeichnet, daß die Trocknung des zweiten Meßgas-Teilstromes durch Abkühlung in einem Meßgaskühler auf eine vorgegebene Taupunkttemperatur erfolgt.Method according to one of claims 1 to 4, characterized in that the drying of the second sample gas partial stream is carried out by cooling in a sample gas cooler to a predetermined dew point temperature. Verfahren nach Anspruch 5, dadurch gekennzeichnet, daß die Taupunkttemperatur 2 °C beträgt.A method according to claim 5, characterized in that the dew point temperature is 2 ° C. Verfahren nach einem der Ansprüche 1 bis 6, dadurch gekennzeichnet, daß eine Druckstabilisierung in der Meßgasleitung über eine Wasservorlage erfolgt, in die eine von der Meßgasleitung vor Teilung des Meßgasstromes abzweigende Stichleitung geführt ist.Method according to one of claims 1 to 6, characterized in that pressure stabilization in the sample gas line takes place via a water reservoir into which a branch line branching off the sample gas line before the sample gas flow is divided is guided. Verfahren nach den Ansprüchen 5 und 7, dadurch gekennzeichnet, daß die Wasservorlage mit Kondensat aus dem Meßgaskühler gespeist wird.Process according to claims 5 and 7, characterized in that the water reservoir is fed with condensate from the sample gas cooler.
EP92121594A 1991-12-20 1992-12-18 Device for determining the moisture content of gases Expired - Lifetime EP0548842B1 (en)

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DE4142118A DE4142118A1 (en) 1991-12-20 1991-12-20 METHOD FOR DETECTING THE HUMIDITY OF GASES
DE4142118 1991-12-20

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DE4433451A1 (en) * 1994-09-20 1996-03-21 Testo Gmbh & Co Method and device for determining the water vapor content in gases
DE102009039183A1 (en) * 2009-08-28 2011-03-17 Thermo- control Körtvélessy GmbH System arrangement for calibration of oxygen measuring probe, has oxygen measuring probe, heated area with defined gas atmosphere, plant control and gas cell for reference air

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DE202009005602U1 (en) 2009-03-24 2009-06-25 Ams Analysen, Mess- Und Systemtechnik Gmbh Device for gas moisture measurement in particular flue gases
DE202009004132U1 (en) 2009-03-24 2009-06-25 Ams Analysen, Mess- Und Systemtechnik Gmbh Device for gas moisture measurement in particular flue gases

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DE102009039183A1 (en) * 2009-08-28 2011-03-17 Thermo- control Körtvélessy GmbH System arrangement for calibration of oxygen measuring probe, has oxygen measuring probe, heated area with defined gas atmosphere, plant control and gas cell for reference air

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DE4142118A1 (en) 1993-06-24
ATE152240T1 (en) 1997-05-15
DE59208396D1 (en) 1997-05-28
ES2103876T3 (en) 1997-10-01
DK0548842T3 (en) 1997-10-27
DE9117010U1 (en) 1995-07-27

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